1. Field of the Invention
The present invention relates to a steering shaft in which a resin-coated layer is formed in a sliding portion and which has a sliding coupling for permitting smooth sliding, and to an apparatus for manufacturing the same.
2. Description of the Related Art
Conventionally, as shown in FIG. 15, a steering shaft is structured so that an axial end portion of an upper shaft U is fitted over an axial end portion of a lower shaft L by adopting a splined structure or an oval, straight-sided cross-sectional structure to form a fitted coupling section so as to allow the upper shaft U to be slidable in the axial direction, and that two injection-molded portions I formed of a synthetic resin are provided between the upper shaft U and the lower shaft U, respectively at an appropriate distance from each end of the fitted coupling section. The injection-molded portions I serve to prevent a clearance in the fitted coupling section of the steering shaft so as to allow the steering shaft of a steering system to be telescopically contractable to secure the safety of the driver during a collision of the vehicle.
A gap is prvided in the fitted coupling section between the lower shaft L and the upper shaft U so that the upper shaft U can be readily positioned or to be movable relative to the lower shaft L. When the lower shaft L and the upper shaft U are fitted, coupled, and fixed to each other by the injection-molded portions I, the injection-molded portions I must be so provided that their axes are concentrically aligned with each other as practically as possible.
However, discharge ports (air vents) are provided in rear surface portions of the upper shaft U (i.e., an outer shaft) in such a manner as to diametrically oppose respective injection ports and form pairs therewith, which oppose injection ports, so as to ensure that a molten synthetic resin, which is cured to form the injection-molded portions I, may be filled sufficiently in forming grooves formed in the lower shaft L (i.e., an inner shaft). For this reason, there are cases where the lower shaft L is pushed in one direction by the ijecting pressure of the molten resin acting in the same direction, so that the lower shaft L is offset within the upper shaft U, and the two shafts are fitted, coupled, and fixed to each other with the axes of the two shafts offset. In such a case, when a load in a bending direction acts on the steering shaft during a collision of the vehicle, there is the possibility of the contracting movement of the steering shaft becoming dull depending on the direction.